1. Field of the Invention
This invention relates generally to a charge pump circuit for a phase lock loop and, more particularly, to a rail-to-rail charge pump circuit that operates as a bi-directional current source to control the voltage applied to a voltage controlled oscillator in a phase lock loop associated with a loop-back self-test circuit.
2. Discussion of the Related Art
Cellular telephone base stations employ several RF transmitter and receiver circuits for processing cellular telephone signals. Cellular telephone signals transmitted from a mobile unit are received by a receiver circuit in the base station, and demodulated and processed therein to decode the signal. The decoded signal is then transferred to a land line or to a transmitter circuit in the base station. The transmitter circuit modulates the information to be transmitted onto a carrier wave for transmission. The transmit and receive signals are typically at a frequency in the range of 800-2000 MHz, where the transmit signal and the receive signals are at different frequencies within a given frequency band with a fixed offset between the signals.
Each receiver circuit typically employs two channels, a primary channel and a diversity channel, each having a separate antenna, so that the receiver circuit can select which of the two receive signals is the strongest for subsequent processing. Some receiver circuits combine the primary channel and diversity channel signals for increased performance. This allows the receiver to be more reliable by lessening the chance that cellular calls are dropped. However, receivers of this type have been limited in their effectiveness for reducing circuit components, while maintaining signal fidelity at high frequencies.
A key function in a cellular telephone system of the type discussed above is the ability to test that the transmitter circuit is operating properly and producing a signal compatible with system requirements. This is commonly done by xe2x80x9cloopingxe2x80x9d a transmit signal back to the receiver circuit in the system to verify that the transmitter and the receiver are operating properly. Because the transmit signal and the receive signal are at different frequencies, a special RF loop-back self-test circuit is required to convert the transmit signal to the receive signal frequency so that the loop-back test can be performed without disturbing the on-going transceiver operation.
Known RF loop-back self-test circuits typically require a separate phase lock loop (PLL) circuit to generate a local oscillator (LO) signal that provides the offset between the transmit signal frequency and the receive signal frequency. The PLL circuit includes various amplifiers and other system components that are compatible with the system requirements. Further, the known self-test circuits require a mixer circuit to convert the signal to an intermediate frequency (IF), or IF to RF. The known loop-back self-test circuits required many integrated circuits and discrete parts, i.e., separate mixers, buffer amplifiers, switches, voltage controlled oscillators, PLLs, to generate the LO signal and switching at significant cost and size. Further, the known self-test circuit designs are typically point designs that do not have the flexibility to change divide ratios and modes of operation to tune the LO frequency by software control for the different frequency offsets between the transmit and receive signals in the many different base stations.
In accordance with the teachings of the present invention, a rail-to-rail charge pump circuit is disclosed that provides a current source and a current sink. The charge pump circuit has particular application for applying or removing charge on a capacitor that controls the voltage applied to a VCO in a phase lock loop used in, for example, a loop-back self-test circuit associated with a transceiver. The charge pump circuit is responsive to two differential logic signals from a phase comparator that compares the phase of a divided down VCO signal to a reference signal. One of the signals from the phase comparator causes the charge pump circuit to provide source current to increase the charge on the capacitor, and the other signal causes the charge pump circuit to provide sink current to decrease the charge on the capacitor.
The charge pump circuit employs complimentary pairs of PNP and NPN bipolar transistors so that it uses relatively low voltage. One of the input signals from the phase comparator is applied to the base terminal of a bipolar transistor to cause it to conduct and generate a mirror current in another bipolar transistor to provide the source current. The other input signal from the phase comparator is applied to the base terminal of a bipolar transistor to cause it to conduct and generate a mirror current in another bipolar transistor to provide the sink current. Therefore, turning on one bipolar transistor provides current flow out of the charge pump circuit, and turning on another bipolar transistor provides current flow into the charge pump circuit. A bleed resistor is coupled to the base terminal of one of the bipolar transistors to barely turn on one of the source current or the sink current so that a small amount of phase error, for example, three degrees, is created. As a result, this small amount of phase error will pull the voltage and phase character of the phase comparator away from a xe2x80x9cDead Zonexe2x80x9d.